Polyoxometalates Paneling through {Mo₂O₂S₂} Coordination: Cation-Directed Conformations and Chemistry of a Supramolecular Hexameric Scaffold

The chemical system based on the [Mo2O2S2(OH2)6]2+ aqua cation (noted L) and the trivacant [AsW9O33]9‑ polyoxometalate (noted POM) has been investigated. Depending upon the ionic strength and the nature of the alkali cations, these complementary components assemble to yield three different architectures derived as hexamer (1), tetramer (2), and dimer (3). This series of clusters displays the same stoichiometry {POM6L9}36‑, {POM4L6}24‑, and {POM2L3}12‑ for 1, 2, and 3, respectively, and their conditions of formation differ mainly by the nature and the concentration of the alkali cation (from Li to Cs). Structural characterizations of 1 reveal a large hexameric supramolecular scaffold (about 25 Å in diameter), which encloses a large internal hole (about 200 Å3) filled by water molecules and alkali cations (Na+ or K+). The hexameric scaffold 1 exhibits a rare flexibility property evidenced in the solid state by two distinct conformations, either eclipsed (1a) or staggered-off (1b). Both conformations appear clearly separated by a large twist angle (∼40°) and depend mainly on the composition of the internal hole. Structure of anion 2 shows a tetrahedral arrangement where the four POM units and the six connecting {Mo2O2S2} linkers are located at the corners and at the edges, respectively. The structure of anion 3 corresponds to the simplest arrangement, described as a dimeric association of two POM units linked by three {Mo2S2O2} pillars. Stability of the hexameric scaffold has been investigated in solution by 183W and 39K NMR and by UV–vis, showing that stability of 1 depends strongly on the proportion of potassium ions, which interfere through host–guest exchange. Density functional methodology (DFT) has been applied to compute the geometries and energies of dimer (3), tetramer (2) and hexamer (1) based on {AsW9O33} (POM) and {Mo2O2S2} (L) units. Calculations tend to show that internal cations act as “glue” to maintain the POM units connected through the conformationally inward-directed {Mo2O2S2} linkers.本研究针对基于[二氧二硫合钼(Ⅱ)六水合阳离子]([Mo2O2S2(OH2)6]2+，记为L)与三缺位多金属氧酸盐(polyoxometalate, POM)[AsW9O33]9‑构成的化学体系展开了探究。根据离子强度与碱金属阳离子的种类差异，这两类互补组分可自组装得到三种不同结构，分别对应六聚体(1)、四聚体(2)与二聚体(3)。该系列团簇的化学计量比分别为{POM6L9}36‑（对应1）、{POM4L6}24‑（对应2）以及{POM2L3}12‑（对应3），其形成条件的差异主要源于碱金属阳离子（从锂到铯）的种类与浓度不同。对六聚体1的结构表征显示，其拥有直径约25 Å的六聚超分子大骨架，内部包含一个约200 Å3的空腔，该空腔由水分子与碱金属阳离子（Na+或K+）填充。六聚体骨架1展现出罕见的柔性特性：在固态下可呈现两种不同构象，即重叠式(1a)与参差式(1b)。两种构象间存在约40°的显著扭转角差异，且其构象主要取决于内部空腔的组分组成。阴离子2的结构为四面体排布，四个POM单元与六个{Mo2O2S2}连接体分别位于四面体的顶点与棱边处。阴离子3的结构为最简单的排布形式，可描述为两个POM单元通过三个{Mo2S2O2}支柱连接形成的二聚体缔合结构。本研究通过183W核磁共振(NMR)、39K核磁共振(NMR)以及紫外-可见(UV–vis)光谱对六聚体骨架在溶液中的稳定性进行了探究，结果表明1的稳定性强烈依赖于钾离子的比例，钾离子可通过主客体交换作用干扰体系稳定性。本研究采用密度泛函理论方法(density functional methodology, DFT)，针对基于{AsW9O33}(POM)与{Mo2O2S2}(L)单元构建的二聚体(3)、四聚体(2)与六聚体(1)的几何结构与能量进行了计算。计算结果表明，内部阳离子可作为“粘合剂”，通过构象向内的{Mo2O2S2}连接体维持POM单元之间的连接。